Conventionally, the volume resistivity (electrical conductivity) of the surface of a thin film of an electronic device or the like has been measured using four microterminals. In the electrical conductivity measurement, four very small probes having a size of several μm to several tens of μm are brought into contact with a target contact portion, and a current is supplied thereto so as to measure the volume resistivity of that portion. In this case, a tunnel current detection method or an optical lever method is used so as to detect contact of the four probes with the target contact portion.
In the tunnel current detection method, a bias voltage is applied between a probe and a target contact portion such that a tunnel current is produced when the probe approaches the target contact portion with a distance of about 1 nm remaining therebetween. Contact of the probe with the target contact portion is detected by detection of the tunnel current. In the optical lever method, as disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. H06-258068, a laser beam is applied to the back surface of a contact portion of a cantilever (corresponding to a probe), and light reflected from the back surface of the cantilever is received by a bi-cell (or tetra-cell) photodetector. Contact between the probe and the target contact portion is detected based on a change in the position where the light is received.
However, the tunnel current detection method has a problem. In the case where an insulating region is formed by, for example, an oxide film naturally formed on a target contact portion, the current flowing between a probe and the target contact portion cannot be detected accurately. In such a case, the probe is strongly pressed against the target contact portion, and the probe and the target contact portion may be damaged. The optical lever method also has a problem. Optical components for causing a laser beam to impinge on the cantilever and causing the reflected light from the cantilever to impinge on the photodetector are necessary, and adjustment of alignment is also necessary. Therefore, the structure of a measuring apparatus becomes complex, and maintenance work for maintaining adequate detection accuracy is very troublesome.